The invention relates to a method and device for the contact-free determination of the condition of a roadway surface, with respect to dryness, wetness or icing, by measuring the reflection of light in the infrared range.
It is known to construct devices which utilize the reflection of radiation to warn of skidding conditions on roads. This can be accomplished, on the one hand, through the emission of radiation in the microwave range by utilizing the different dielectric constants of water and ice (see DE 29 12 645-A1), but also by the reflection of infrared radiation (IR radiation) (see DE 2,712,199-B1, EP 0,005,696-A1 and DE 3,023,444-A1).
DE 2,712,199-B1 and EP 0,005,696-A1 disclose a device for warning of skidding conditions on roadways. In this device, IR radiation, which is produced by two laser diodes, is transmitted by way of IR wave guides into the vicinity of the road surface and from there it is in turn directed to the common receiver by way of IR wave guides.
The wavelength of the measuring light pulses thus lies, at an absorption maximum for ice, in the wavelength regions of 790 to 810 nm, 890 to 920 nm, 1020 to 1060 nm or 1260 to 1290 nm. The second laser diode radiates reference beam pulses whose wavelengths lie in the vicinity of the absorption maximum for ice, but must not be affected by ice nor water nor water vapor.
DE 3,023,444-A1 discloses a device which, by means of a light projector transmits onto the roadway surface IR radiation in a range from 1400 to 2500 nm, in which snow demonstrates less reflectivity than a dry road surface. Three modified arrangements are recommended for determining roadway conditions:
One configuration is based on an IR radiation source in a range from 1400 to 2500 nm and employs two sensors in the IR range of which the first detects mirror reflected light and the second reflected scattered light on the surface. Additionally, the temperature is measured with a thermometer.
The second configuration dispenses with the thermometer and the light within the range of the visible spectrum and in the IR range between 1400 and 2500 nm is beamed onto the surface. Two sensors serve to measure the reflected radiation in the visible and in the IR range.
The third configuration also makes its determination by means of a light source which emits IR radiation in a range from 1400 to 2500 nm and visible light in a range from 500 to 1090 nm. is equipped with a detector for scattered IR light, a detector for scattered light in the visible range, a detector for mirror reflected light in the visible range and a temperature measuring device.
In all three configurations, the signals measured with the individual sensors are compared by way of logic circuits in an evaluation unit having stored patterns of reference signals in order to specify the road conditions.
Determination of the surface condition (dry, wet, icy) of a roadway is of extreme significance for traffic safety.
Sensor systems already exist which require the sensor to contact the surface to be examined, and others that operate without contact, which are based on reflection measurements. The existing solutions are structurally elaborate and expensive and the detection reliability of these systems is limited.
The devices disclosed in DE 3,023,444-A1 which also use broad-band IR radiation for irradiation are very expensive with respect to the devices required for signal pattern comparison. In particular, a comparison using previously stored signal patterns is made.
The first embodiment listed there, in which IR radiation in the wavelength region from 1400 to 2500 nm is used, permits a differentiation between a wet or icy road condition only by way of an additional temperature measurement. The other embodiments, in addition to radiation of light in the IR range from 1400 to 2500 nm, also contain light components in the visible range from 500 to 1090 nm, which are detected by separate detectors as scattered and mirror reflected light. Under certain conditions, this could result in transverse sensitivities due to the incidence of sunlight.
The device according to DE 2,712,199-B1 and EP 0,005,696-A1 employs two pulse light sources (laser diodes), but only one detector, so that the measurement can only be performed sequentially.
Since the selection of wavelengths is already made before the surface is struck, and the radiation is not filtered at the input of the detector, interference may also occur here due to the radiation from further light sources. This is to be prevented by the radiation of reference radiation pulses. However, the wavelength of these pulses is selected such that the reflection is specifically influenced neither by ice nor water. Accordingly, this would involve a wavelength at which ice and water exhibit the same reflectivity, which is, in addition, as independent as possible of different layer thicknesses. The wavelength of the measuring light pulse lies within an absorption maxima for ice. However, there is only a minor difference between the absorption maxima for ice and water in the preferably used wavelength range, so that an adequate wavelength stability of the measuring light pulses must be ensured. This is critical in laser diodes, because they exhibit a not insignificant wavelength drift, depending on the temperature. If laser diodes are used, costly temperature stabilization is therefore necessary.
In the devices according to EP 0,005,691-A1 and DE 2,712,199-B1 filters are additionally used in the radiation path. However, with strong drifts the result of this is that only minimal or no light intensity remains available for reflection on the surface. Furthermore, if the drift of the two diodes is not uniform, falsification of the resulting signal levels might occur.